In the mining industry, mined ores and coal are upgraded using appropriate separation method. They are usually crushed and/or pulverized to detach (or liberate) the valuable components from waste rocks prior to subjecting them to appropriate solid-solid separation methods. Although coal is not usually pulverized as finely as ores, a significant portion of a crushed coal is present as fines. Froth flotation is the most widely used method of separating the valuables from valueless present in the fines. In this process, the fine particles are dispersed in water and small air bubbles are introduced to the slurry, so that hydrophobic particles are selectively collected on the surface of the air bubbles and exit the slurry while hydrophilic particles are left behind.
A small dose of surfactants, known as collectors, are usually added to the aqueous slurry to render one type (or group) of particles hydrophobic, leaving others unaffected. For the case of processing high-rank coals, no collectors are necessary as the coal is naturally hydrophobic. When the coal particles are not sufficiently hydrophobic, however, hydrocarbon oils such as diesel oil or kerosene are added to enhance their hydrophobicity.
It has been shown recently that air bubbles are hydrophobic (Yoon and Aksoy, J. Colloid and Interface Science, vol. 211, pp. 1-10, 1999). It is believed, therefore, that air bubbles and hydrophobic particles are attracted to each other by hydrophobic interaction.
The floated products, which are usually the valuables, are in the form of aqueous slurry, typically in the range of 10 to 35% solids. They are dewatered frequently by filtration prior to further processing or shipping to consumers. The process of dewatering is often described by means of the Laplace equation:                                           Δ            ⁢                          xe2x80x83                        ⁢            p                    =                                    2              ⁢                              xe2x80x83                            ⁢                              γ                23                            ⁢              cos              ⁢                              xe2x80x83                            ⁢              θ                        r                          ,                            [        1        ]            
in which r is the average radius of the capillaries formed in between the particles that make up a filter cake, xcex94p the pressure of the water inside the capillaries, xcex323 the surface tension at the water(3)-air(2) interface and xcex8 is the contact angle of the particles constituting the filter cake. The capillary water can be removed when the pressure drop applied across the cake during the process of filtration exceeds xcex94p. Thus, a decrease in xcex323 and xcex8, and an increase in r should help decrease xcex94p and thereby facilitate the process of dewatering.
The U.S. Pat. No. 5,670,056 disclosed a method of using hydrophobizing agents that can increase the contact angle (xcex8) above 65xc2x0 and, thereby, facilitate dewatering processes. Mono-unsaturated fatty esters, fatty esters whose hydruphile-lipophile balance (HLB) numbers are less than 10, and water-soluble polymethylhydrosiloxanes were used as hydrophobizing agents. More recently, a series of U.S. patents have been applied for to disclose the methods of using a group of nonionic surfactants with HLB numbers in the range of 1 to 15, See U.S. patent application Ser. No. 09/368,945, entitled xe2x80x9cMethods for Using Modified Natural Products as Dewatering Aids for Fine Particles, naturally occurring lipids, U.S. patent application Ser. No. 09/326,330, filed Jun. 7, 1999, and a U.S. patent application filed Mar. 21, 2000 and entitled, xe2x80x9cMethods of Improving Centrifugal Filtrationxe2x80x9d by H. Yoon on modified lipids to increase xcex8 beyond the level that can normally be achieved using flotation collectors and, hence, improve dewatering. The contents of the above three patent applications are hereby incorporated herein by reference.
Ever since the flotation technology was introduced to the mining industry, its practitioners have been seeking for appropriate collectors that can increase xcex8 as much as possible without causing unwanted minerals inadvertently hydrophobic. A theoretical model developed by Mao and Yoon (International Journal of Mineral Processing, vol. 50, pp. 171-181, 1996) showed that an increase in xcex8 can increase the rate at which air bubbles can collect hydrophobic particles.
From the foregoing, it should be apparent to the reader that one obvious object of the present invention is the provision of novel methods of enhancing the hydrophobicity of the particles to be floated beyond the level that can be achieved using collectors, so that the rate of bubble-particle attachment and, hence, the rate of flotation can be increased.
Another important objective of the invention is the provision of increasing the hydrophobicity difference between the particles to be floated and those that are not to be floated, so that the selectivity of the flotation process can be increased.
An additional objective of the present invention is the provision of increasing the hydrophobicity of the particles that are usually difficult to be floated such as coarse particles, ultrafine particles, oxidized particles, and the particles that are difficult to be floated in solutions containing high levels of dissolved ions.
Still another object of the present invention is the provision of a novel collector for the flotation of phosphate minerals that are more effective than the fatty acids that are most commonly used today.
The present invention discloses methods of increasing the rate of flotation, in which air bubbles are used to separate hydrophobic particles from hydrophilic particles. In this process, the hydrophobic particles adhere on the surface of the air bubbles and subsequently rise to the surface of the flotation pulp, while hydrophilic particles not collected by the air bubbles remain in the pulp. Since air bubbles are hydrophobic, the driving force for the bubble-particle adhesion may be the hydrophobic attraction. Therefore, one can improve the rate of bubble-particle adhesion and, hence, the rate of flotation by increasing the hydrophobicity of the particles to be floated.
In conventional flotation processes, appropriate collectors (mostly surfactants) are used to render selected particles hydrophobic. The collector molecules adsorb on the surface of the particles with their polar groups serving effectively as xe2x80x98anchorsxe2x80x99, leaving the hydrocarbon tails (or hydrophobes) exposed to the aqueous phase. Since the hydrocarbon tails are hydrophobic, the collector-coated surfaces acquire hydrophobicity, which is a prerequisite for flotation. In general, the higher the packing density of the hydrophobes on a surface, the stronger the surface hydrophobicity.
A conventional measure of hydrophobicity is water contact angle (xcex8). Thermodynamically, the higher the contact angle, the more favorable the flotation becomes. Therefore, there is a need to increase the hydrophobicity as much as possible. Unfortunately, collector coatings do not often result in the formation of close-packed monolayers of hydrophobes. The polar groups of collector molecules can adsorb only on certain sites of the surface of a particle, while the site density does not usually allow formation of close-packed monolayers of hydrophobes.
It has been found in the present invention that certain groups of reagents can be used in addition to collectors to further increase the packing density of hydrophobes and, thereby, enhance the hydrophobicity of the particles to be floated. Four groups of reagents have been identified. These include nonionic surfactants of low HLB numbers, naturally occurring lipids, modified lipids, and hydrophobic polymers. These reagents, having no highly polar groups in their molecules, can adsorb in between the hydrocarbon chains of the collector molecules adsorbed on the surface of particles. Most of the hydrophobicity-enhancing reagents used in the present invention are insoluble in water, in which case appropriate solvents may be used to carry the reagents and spread them on the surface. However, some of the reagents may be used directly without solvents.
The solvents for the hydrophobicity-enhancing reagents may include but not limited to short-chain aliphatic hydrocarbons, aromatic hydrocarbons, light hydrocarbon oils, glycols, glycol ethers, ketones, short-chain alcohols, ethers, petroleum ethers, petroleum distillates, naphtha, glycerols, chlorinated hydrocarbons, carbon tetrachloride, carbon disulfide, and polar aprotic solvents such as dimethyl sulfoxide, dimetyl formamide, and N-methyl pyrrolidone. The amounts of solvents required vary depending on the type of hydrophobicity-enhancing reagents and the type of solvents used.
In the flotation industry, different types of collectors are used for different minerals. For the flotation of sulfide minerals, thiol-type collectors are used. For the flotation of oxide minerals, high HLB surfactants are used. For the flotation of naturally hydrophobic coal and minerals, hydrocarbon oils such as fuel oils are used. The hydrophobicity-enhancing reagents disclosed in the present invention can be used for any type of minerals, because these reagents interact primarily with the hydrocarbon chains of the collector molecules adsorbed on the surface.
The benefits of using the hydrophobicity-enhancing reagents can be seen with all types of particles present in a flotation cell. However, the most significant improvements can be obtained with the particles that are either too small or too large to be floated. For the case of minerals, it is difficult to float particles smaller than 0.01 mm and larger than 0.15 mm. The novel hydrophobicity-enhancing reagents are also useful for the flotation of minerals that have become considerably hydrophilic due to oxidation.
In the phosphate minerals industry, fatty acids are commonly used as collectors. However, their efficiency deteriorates when the plant water contains high levels of phosphate ions. This problem can be readily overcome by using the novel hydrophobicity-enhancing reagents disclosed in the present invention in addition to a small amount of fatty acids. It has been found also that phosphate esters can be used as standalone collectors for phosphate minerals. These new collectors are effective in solutions containing high levels of dissolved phosphate ions.